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|back_and_app_list_background_| is interfering with event targeting and
causing the app list button to be unresponsive, specifically in the
right side of the button. NOTE: This only repros on device, not on the
emulator.

Prevent |back_and_app_list_background_| from recieving events to prevent
this.

Bug: 900632
Change-Id: Ib2cd8e51cb560d84307904c86ae46ecc388e2424
Reviewed-on: https://chromium-review.googlesource.com/c/1338723Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608796}